Among various processes of polymerization reactions, solution polymerization process is typically applied to synthesize cyclic olefins polymers, in which monomers, catalysts, and polymers are all dissolved in a solvent. The reaction begins with the monomers and the catalysts dissolved in a solvent in a reactor. With the activated catalysts the monomers are continuously added to the growing polymer chain at the catalyst active site by coordinate covalent bonding. The dissolution of the polymer in a solvent is maintained to perform the reaction in a single homogeneous liquid phase. Since the amount of polymer is gradually increased in the solution during the reaction, the viscosity of the solution is continuously increased with the reaction.
When the reaction is finished, the solution in a reactor becomes a mixture containing the polymer obtained by the synthesis, unreacted monomers, the solvent, and a small amount of catalyst. Accordingly, after the reaction, a process for selectively recovering the polymer from the solution is required. In this step, an additional solvent may be added into the reactor in order to reduce the viscosity, thus easily transporting the solution to perform post-treatment.
Generally, two different methods are commercially available to selectively recover the polymer from the solvent and the unreacted monomers in the obtained liquid phase polymer solution from solution polymerization process. Firstly, the polymer solution is dispersed as liquid drops over a large amount of liquid phase dispersion medium such as water under a condition of temperature that is the boiling point temperature of the monomer or the solvent or higher so that the residual monomers and solvent are vaporized to give coarse solid polymer particles (usually called “crumb”). Since the obtained polymer particles are wet with water, further drying process is required to get final dry polymer particles. This method, so called steam stripping process, is commercially used to recover thermo-sensitive polymers such as butadiene rubber or styrene-butadiene elastomers from the polymer solution after the polymerization reaction.
In the second method, a polymer solution is processed under the condition of high temperature and reduced pressure so that residual monomers and solvent are vaporized from the molten polymer. This method, so called devolatilization, thus requires high temperature which is identical with or higher than the melting point of the polymer and a pressure reducing device such as vacuum pump. It is commercially used to produce melt-processable polymers such as polystyrene and styrene-acrylonitrile copolymers.
Meanwhile, for cyclic olefin polymers whose monomers typically show very high boiling point temperature (150° C. or more) and polymers not having a specific melting point temperature, both of the above-mentioned methods cannot be used. In this case, a method of using the difference in solubility of polymers with respect to various organic solvents is used, and generally, a solid polymer is recovered from a polymer solution obtained after polymerization by using a precipitation phenomenon that a polymer dissolved in a solvent precipitates out in a solid form when the antisolvent having a very low solubility of polymer is added to the polymer solution.
The precipitation phenomenon means that when a polymer material dissolved in a liquid solvent is added to an antisolvent which cannot dissolve the polymer material, the polymer material is precipitated out in a solid form. The antisolvent is a material which can be mixed with the solvent of the polymer solution and has a low solubility with respect to the polymer material to be separated.
If the antisolvent is used in a sufficiently large amount, the polymer material is precipitated while coming into contact with the antisolvent. Accordingly, the precipitation phenomenon relates to a phase-conversion reaction where liquid polymer phase is rapidly converted into solid phase.
In the case of the cyclic olefin resin, the precipitation phenomenon is used in order to recover the polymer in the polymer solution synthesized after the solution polymerization.
A batch type dropping method is known in the related art. Examples of the dropping method may include a forward dropping method where a polymerization solution is added to an antisolvent as a liquid droplet, and a backward dropping method where an antisolvent is added to a polymerization solution as a liquid droplet.
In a cyclic olefin polymerization process, the above-mentioned dropping precipitation method is used and a single kind of antisolvent for complete precipitation or two kinds of antisolvents for partial precipitation followed by complete precipitation are used to perform the process.
The reason why two or more kinds of antisolvents are used during the dropping method is that it is easy to obtain the polymer in a particle form as compared to the case of a single kind of antisolvent.
First, the antisolvent for partial precipitation is added to the polymerization solution to partially precipitate the polymer. At this time, the polymerization solution is changed from a clear and transparent liquid state to a semi-transparent and frosty state. Next, a large amount of antisolvent for complete precipitation is added to completely precipitate the polymer particles.
The polymer which is dissolved in the polymerization solution is precipitated out in a solid form if the antisolvent is added thereto. In respect to the dropping method, very small solid particles are first formed during the partial precipitation, and the formed solid particles coagulate to each other to form larger size particles when the antisolvent for complete precipitation is further added.
In this case, it is presumed that the particle size is determined depending on the supply rate of the antisolvent and the rpm of an impeller in a precipitation reactor or the tip speed of the impeller.
A repulsive force against a cohesive force between particles is generated due to a shearing force from the impeller. Accordingly, if the cohesive force between particles is larger than the repulsive force generated due to the shearing force of the impeller, the particle size will be continuously increased.
Meanwhile, as the precipitation of particles reaches the complete precipitation, the cohesive force between particles is decreased. When particles have been completely precipitated, the cohesive force is hardly generated between particles since salvation effect by solvent is blocked by antisolvent.
In a dropping method, however, an antisolvent is gradually supplied onto the surface of a polymer solution, while high shearing force is generated near the impeller and a relatively low shearing force is generated on the surface of the solution.
Therefore, if particles are strongly cohered on the surface of the solution, there is a possibility that the polymer is obtained in the form of not particles but a cake.